Now it depends on what you want from the car... you know you can't actually compare those cars, the M4 with an X6 35i... I wanted to go higher in class, I was tired of all of the sound and stiffness of the suspension so this is why I have moved from the M4.
Suspension : I was almost every time driving the M4 with the suspension in comfort mode, unfortunately I can't set it up like I was able to do it in the M4 so I always drive the X6 also in comfort mode (dealer said that in comfort mode way of driving also the suspension will be in comfort mode) except for some cases when I push it. The X6 feels just like a boat that I am going to lose control of, that I have never felt with the M4. Anyway, this is exactly what I wanted, a big boat that won't kill my back anymore on bad roads.
Handling : I was always keeping my steering setting in sports mode and I liked the feeling of it a lot, the X6 does not give that handling feeling like the M4 did.
Acceleration : Can't compare the two again, the M4 had huge reserves of power, every time the car would have pushed you like crazy, but the X6 is not bad at all, it has a decent acceleration judging by its size and weight.
Disclaimer : By no means I can compare the two cars, for the moment the X6 will better serve my needs, I will go back to the M division in 2-3 years maximum and probably I will get an M6 GC or an X6 M if I will still like the X6 so much.